PSR B1937 Research Articles

An Estimation of the Initial Period of Pulsars

The initial period of a pulsar is an important factor in our understanding of the formation of neutron stars and of the nature of the equation of state of neutron star matter. Up to now this quantity can only be obtained for a few pulsars for which accurate age and braking index are known. Based on the theory of the off-center dipole emission, in which pulsars obtain their high velocities depending on the initial periods, we calculate the initial period using the proper motion data. Because the orbital velocity of the progenitor and asymmetric kick in the supernova explosion may also contribute to the observed velocity of the pulsar, the derived values of initial periods are lower limits. For normal pulsars, the initial periods are in the range of 0.6 similar to 2.6 ms. For the millisecond pulsars, the initial periods are comparable to their current periods, and the ratio between the initial period and the current period increases with the decrease of the current period. For PSR B1937+21 with the shortest period of 1.56 ms, the ratio is 0.77.

X-ray and gamma-ray emission from millisecond pulsars

We present a self-consistent model to describe X-ray and γ-ray emission from millisecond pulsars (MSPs). The X-rays of MSPs are produced by the backflow of primary charged particles from the outer gap and most likely consist of three components, two thermal components and one power law component if there is a strong multipole magnetic field on the stellar surface. The backflow of ultra-relativistic particles emits photons with energies about several tens of GeV via curvature radiation. These photons cause an electromagnetic cascade about 2–3 stellar radii above the polar cap. The synchrotron radiation of these cascade e pairs produces hard X-rays with a power law index ~. Near 105 cm above the stellar surface, the primary charged particles encounter the strong surface magnetic field, which alters the local radius of curvature greatly, and they quickly loose more than half of their remaining energies to curvature radiation. These curvature photons heat up the polar cap area with a radius ~ 105 cm, which produce the softer thermal X-ray component. Finally, the primary charged particles deposit their remaining energies in a much smaller polar cap area, which corresponds to the footprints of outer gap and produce the medium hard X-ray component. γ-rays are produced in the outer gap through synchro-curvature radiation. We have applied this model to the MSPs which emit pulsed X-rays and likely γ-rays such as PSR J0437-4715, PSR J2124-3358, PSR J0218+4232 and PSR B1821-24. Our results give an agreement between predicted spectrum and the observed spectrum of MSP emission.

High-resolution single-pulse studies of the Vela pulsar

We present high-resolution multi-frequency single-pulse observations of the Vela pulsar, PSR B0833 − 45, aimed at studying micro-structure, phase-resolved intensity fluctuations and energy distributions at 1.41 and 2.30 GHz. We find micro-structure in about 80 per cent of all pulses independent of observing frequency. The width of a micro-pulse seems to depend on its peak flux density, whilst quasi-periodicities observed in micro-pulses remain constant. The width of the micro-pulses may decrease with increasing frequency, but confirmation is needed at higher frequencies. The fraction of pulses showing quasi-periodic micro-pulses may become smaller at higher frequencies. We show that the micro-pulse width in pulsars has a period dependence which suggests a model of sweeping micro-beams as the origin of the micro-pulses. Like individual pulses, the micro-pulses of Vela are highly elliptically polarized. There is a strong correlation between Stokes parameters V and I in the micro-structure. We do not observe any micro-pulses where the handedness of the circular intensity changes within a micro-pulse, although flips within a pulse are not uncommon. We show that the V/I distribution is Gaussian with a narrow width and that this width appears to be constant as a function of pulse phase. The phase-resolved intensity distributions of I are best fitted with log-normal statistics. Extra emission components, i.e. ‘bump’ and ‘giant micro-pulses’, discovered by Johnston et al. at 0.6 and 1.4 GHz are also present at the higher frequency of 2.3 GHz. The bump component seems to be an extra component superposed on the main pulse profile but does not appear periodically. The giant micro-pulses are time-resolved and have significant jitter in their arrival times. Their flux density distribution is best fitted by a power-law, indicating a link between these features and ‘classical’ giant pulses as observed for the Crab pulsar (PSR B0531 + 21), PSR B1937 + 21 and PSR B1821 − 24. We find that Vela contains a mixture of emission properties representing both ‘classical’ properties of radio pulsars (e.g. micro-structure, high degree of polarization, S-like position angle swing, orthogonal modes) and features which are most likely related to high-energy emission (e.g. extra profile components, giant micro-pulses). It hence represents an ideal test case to study the relationship between radio and high-energy emission in significant detail.

The role of multipolar magnetic fields in pulsar magnetospheres

We explore the role of complex multipolar magnetic fields in determining physical processes near the surface of rotation powered pulsars. We model the actual magnetic field as the sum of global dipolar and star-centred multipolar fields. In configurations involving axisymmetric and uniform multipolar fields, ‘neutral points’ and ‘neutral lines’ exist close to the stellar surface. Also, the curvature radii of magnetic field lines near the stellar surface can never be smaller than the stellar radius, even for very high-order multipoles. Consequently, such configurations are unable to provide an efficient pair-creation process above pulsar polar caps, necessary for plasma mechanisms of generation of pulsar radiation. In configurations involving axisymmetric and non-uniform multipoles, the periphery of the pulsar polar cap becomes fragmented into symmetrically distributed narrow subregions where curvature radii of complex magnetic field lines are less than the radius of the star. The pair-production process is only possible just above these ‘favourable’ subregions. As a result, the pair plasma flow is confined within narrow filaments regularly distributed around the margin of the open magnetic flux tube. Such a magnetic topology allows us to model the system of 20 isolated subbeams observed in PSR B0943+10 by Deshpande & Rankin. We suggest a physical mechanism for the generation of pulsar radio emission in the ensemble of finite subbeams, based on specific instabilities. We propose an explanation for the subpulse drift phenomenon observed in some long-period pulsars.

Possible Evidence for Pulsed X‐Ray Emission from the Outer Gap in PSR B1937+21

The fastest millisecond pulsar PSR B1937+21 presents an interpulse separated from the main pulse by nearly 180° at radio frequencies. Recently, the ASCA observations detected pulsed X-ray emission from this pulsar. Only a single narrow X-ray pulse is observed, which is coincident with the radio interpulse in phase. We investigate the possible origin of the pulsed X-rays from the polar cap (PC) accelerators or the outer gap (OG) accelerators in the frame of a PC model and an OG model, respectively, by assuming a dipolar magnetic field structure and the same radio emission pattern from its poles for the pulsar. The OG model can naturally explain the main observational facts. For the PC model, the coincidence between the X-ray pulse and the radio interpulse cannot be reproduced in the assumed case. However, when considering possible deviation from our assumption, the PC model may still be valid for this pulsar in some cases.

Vacuum gap model for PSR B0943+10

PSR B0943+10 is known to show remarkably stable drifting subpulses, which can be interpreted in terms of a circumferential motion of 20 sparks, each completing one circulation around the periphery of the polar cap in 37 pulsar periods. We use this observational constraint and argue that the vacuum gap model can adequately describe the observed drift patterns. Further we demonstrate that only the presence of strong non-dipolar surface magnetic field can favor such vacuum gap formation. Subsequently, for the first time we are able to constrain the parameters of the surface magnetic field, and model the expected magnetic structure on the polar cap of PSR B0943+10 considering the inverse Compton scattering photon dominated vacuum gap.

Giant radio pulses from the millisecond pulsar PSR B1937+21 at 327 MHz

Seven giant radio pulses were recorded from the millisecond pulsar PSR B1937+21 during $\approx$ 8.1 minutes observation by the Ooty Radio Telescope (ORT) at 326.5 MHz. Although sparse, these observations support most of the giant pulse behavior reported at higher radio frequencies (430 to 2380 MHz). Within the main componentof the integrated profile, they are emitted only in a narrow ($\la$ 47 $\mu$s) window of pulse phase, close to its peak. This has important implications for doing super-high precision timing of PSR B1937+21 at low radio frequencies.

A search for giant pulses in Vela-like pulsars

We have carried out a survey for ‘giant pulses’ in six young, Vela-like pulsars. In no cases did we find single pulses with flux densities more than 10 times the mean flux density. However, in PSR B1706–44 we have detected giant micro-pulses very similar to those seen in the Vela pulsar. In PSR B1706–44 these giant micro-pulses appear on the trailing edge of the profile and have an intrinsic width of ∼1 ms. The cumulative probability distribution of their intensities is best described by a power law. If the power law continues to higher intensities, then 3.7×106 rotations are required to obtain a pulse with 20× the mean pulse flux. This number is similar to the giant pulse rate in PSR B1937+21 and PSR B1821–24 but significantly higher than that for the Crab.

Giant Pulses from the Millisecond Pulsar B1821−24

We have carried out a survey for `giant pulses' in 5 millisecond pulsars. We detect individual pulses from the high Edot pulsar PSR B1821-24 with energies exceeding 50x the mean pulse energy. These giant pulses are concentrated in a narrow phase window coincident with the power-law non-thermal pulse seen in hard X-rays. This is the third example of the giant pulse phenomenon. It supports the idea that large B fields in the outer magnetosphere are critical to the formation of such pulses and further suggests a direct connection between giant pulses and high energy emission.

Pulsed X‐Ray Emission from the Fastest Millisecond Pulsar: PSR B1937+21 withASCA

We have detected pulsed X-ray emission from the fastest millisecond pulsar known, PSR B1937+21 (P = 1.558 ms), with ASCA. The pulsar is detected as a point source above ~1.7 keV, with no indication of nebulosity. The source flux in the 2-10 keV band is found to be f = (3.7 ± 0.6) × 10-13 ergs s-1 cm-2, which implies an isotropic luminosity of LX = 4πD2f ~ (5.7 ± 1.0) × 1032(D/3.6 kpc)2 ergs s-1, where D is the distance, and an X-ray efficiency of ~5 × 10-4, relative to the spin-down power of the pulsar. The pulsation is found at the period predicted by the radio ephemeris with a very narrow primary peak, the width of which is about 1/16 phase (~100 μs), near the time resolution limit (61 μs) of the observation. The instantaneous flux in the primary peak (1/16 phase interval) is found to be (4.0 ± 0.8) × 10-12 ergs s-1 cm-2. Although there is an indication for the secondary peak, we consider its statistical significance too low to claim a definite detection. The narrow pulse profile and the detection in the 2-10 keV band imply that the X-ray emission is caused by the magnetospheric particle acceleration. Comparison of X-ray and radio arrival times of pulses indicates, within the timing errors, that the X-ray pulse is coincident with the radio interpulse.

A search for the optical counterpart to PSR B1821-24 in M 28

We have analysed archival HST/WFPC2 images in both the F555W & F814W bands of the core field of the globular cluster M 28 in an attempt to identify the optical counterpart of the magnetospherically active millisecond pulsar PSR B1821-24. Examination of the radio derived error circle yielded several potential candidates, down to a magnitude of V ~ 24.5 (V 0 ~ 23.0). Each were further investigated, both in the context of the CMD of M 28, and also with regard to phenomenological models of pulsar magnetospheric emission. The latter was based on both luminosity-spindown correlations and known spectral flux density behaviour in this regime from the small population of optical pulsars observed to date. None of the potential candidates exhibited emission expected from a magnetospherically active pulsar. The fact that the magnetic field & spin coupling for PSR B1821-24 is of a similar magnitude to that of the Crab pulsar in the vicinity of the light cylinder has suggested that the millisecond pulsar may well be an efficient nonthermal emitter. ASCA's detection of a strong synchrotron-dominated X-ray pulse fraction encourages such a viewpoint. We argue that only future dedicated 2-d high speed photometry observations of the radio error-circle can finally resolve this matter.

Unusual features of the millisecond pulsar 1937 + 21 at 406.9 MHz

Observations of the millisecond pulsar PSR B1937 + 21 acquired in Summer 1997 with the Medicina cross telescope revealed two interesting features: the existence of long-lived microstructures persisting over several minutes, and a close relationship between the pulsar flux and pulse arrival times, due to refraction on a discrete inhomogeneity. The latter effect, though observed earlier at higher frequencies by the Nancay group, is rather peculiar at our low frequency.

Single‐Pulse Characteristics of the Millisecond Radio Pulsar PSR B1937+21 at 430 MHz

The single-pulse characteristics of the millisecond pulsar PSR B1937+21 are studied using the recently installed Caltech baseband recorder at the Arecibo Radio Observatory in Puerto Rico. This is the first such analysis of this object that includes both average intensity pulses as well as giant pulses. Pulse ensemble-averaging techniques are developed in order to study the characteristics of PSR B1937+21's single pulses since the high time resolution signal-to-noise ratio is less than unity. This analysis reveals that the non-giant pulse radio emission is extremely stable. All observed fluctuations are consistent with diffractive interstellar scattering. Such intrinsic stability has yet to be observed in other radio pulsars.

Multifrequency Observations of Giant Radio Pulses from the Millisecond Pulsar B1937+21

Giant pulses are short, intense outbursts of radio emission with a power-law intensity distribution that have been observed from the Crab pulsar and PSR B1937+21. We have undertaken a systematic study of giant pulses from PSR B1937+21 using the Arecibo telescope at 430, 1420, and 2380 MHz. At 430 MHz, interstellar scattering broadens giant pulses to durations of ~50 μs, but at higher frequencies the pulses are very short, typically lasting only ~1-2 μs. At each frequency, giant pulses are emitted only in narrow (10 μs) windows of pulse phase located ~55-70 μs after the main and interpulse peaks. Although some pulse-to-pulse jitter in arrival times is observed, the mean arrival phase appears stable; a timing analysis of the giant pulses yields precision competitive with the best average profile timing studies. We have measured the intensity distribution of the giant pulses, confirming a roughly power-law distribution with approximate index of -1.8, contributing 0.1% to the total flux at each frequency. We also find that the intensity of giant pulses falls off with a slightly steeper power of frequency than the ordinary radio emission.

The Extreme Scattering Event toward PKS 1741−038: VLBI Images

We report multiepoch VLBI observations of the source PKS 1741-038 (OT 068) as it underwent an extreme scattering event (ESE). Observations at four epochs were obtained, and images were produced at three of these. One of these three epochs was when the source was near the minimum flux density of the ESE, the other two were as the flux density of the source was returning to its nominal value. The fourth epoch was at the maximum flux density during the egress from the ESE, but the VLBI observations had too few stations to produce an image. During the event the source consisted of a dominant, compact component, essentially identical to the structure seen outside the event. However, the source's diameter increased slightly at 13 cm, from near 0.6 mas outside the ESE to near 1 mas during the ESE. An increase in the source's diameter is inconsistent with a simple refractive model in which a smooth refractive lens drifted across the line of sight to PKS 1741-038. We also see no evidence for ESE-induced substructure within the source or the formation of multiple images, as would occur in a strongly refractive lens. However, a model in which the decrease in flux density during the ESE occurs solely because of stochastic broadening within the lens requires a larger broadening diameter during the event than is observed. Thus, the ESE toward PKS 1741-038 involved both stochastic broadening and refractive defocusing within the lens. If the structure responsible for the ESE has a size of order 1 AU, the level of scattering within an ESE lens may be a factor of 107 larger than that in the ambient medium. A filamentary structure could reduce the difference between the strength of scattering in the lens and ambient medium, but there is no evidence for a refractively induced elongation of the source. We conclude that, if ESEs arise from filamentary structures, they occur when the filamentary structures are seen lengthwise. We are able to predict the amount of pulse broadening that would result from a comparable lens passing in front of a pulsar. The pulse broadening would be no more than 1.1 μs, consistent with the lack of pulse broadening detected during ESEs toward the pulsars PSR B1937+21 and PSR J1643-1224. The line of sight toward PKS 1741-038 is consistent with a turbulent origin for the structures responsible for ESEs. The source PKS 1741-038 lies near the radio Loop I and is seen through a local minimum in 100 μm emission.

High-Precision NBPP Timing Measurements at Nançay

The Nançay radiotelescope in France is a large collecting area (7000m2) with receivers around 1.4, 1.7 and 3.5GHz. At this observatory, we are conducting frequent high-precision timing observations of 5 millisecond pulsars (PSR B1937+21, B1821-24 since 1988; J1643-1224, J1713+0747 and B1620-26 since 1996) with a swept frequency oscilllator based on a DDS as a dedisperser.The most interesting result from these dense series of observations is the detection of several Extreme Scattering Events in direction of B1937+21 and possibly in direction of B1821-24 (Cognard, 1993,Nature,366, 320; Cognard & Lestrade, 1996, in ASP Conf. Ser. Vol 105, Pulsars: Problems and Progress (San Francisco: ASP), 469; Lestrade, Rickett & Cognard 1998, A&A,334, 1068). The flux density variations and TOA fluctuations observed have been used to estimate the size (several AU) and electron density (a few hundreds electrons cm−3) of the discrete ionised clouds localized in the interstellar medium that are thought to be responsible for this phenomena. The number of events recorded at Nançay in direction of B1937+21 yields the space density 105– 106pc−3for these clouds. This density is very large and is about 100 times higher than the density estimated from Extreme Scattering Events observed in direction of extragalactic radio sources (Fiedler et al., 1994,ApJ,430, 581). This might mean that the line of sight to B1937+21 is peculiar. This is being investigated with additional observations of a larger array of pulsars at Nançay with the Navy-Berkeley-Pulsar Processor (Figure 1) This processor NBPP (Foster et al., in ASP Conf. Ser. Vol 105, Pulsars: Problems and Progress (San Francisco: ASP), 25) has been used to acquire pulsar search data for 2 years at Nancay and we are now using its real-time folding capability.

Detection of Pulsed X-ray Emission from The Fastest Millisecond Pulsar PSR B1937+21 with ASCA

AbstractWe report the first detection of the pulsed X-ray emission from the fastest millisecond pulsar known PSR B1937+21 (P=1.558msec) with ASCA. The pulsar is detected as a point source above ∼ 1.7keV, with no nebulosity indicated. The source flux in the energy band 2–10keV is found to bef= 4.3 × 10−13erg s−1cm−2, which corresponds to the luminosity ofLx= 4πD2f≈ 6.6 × 1032(D/3.6kpc)2erg s−1, whereDis the distance, and correspond to ≈ 6 × 10−4of the rotation power of the pulsar. The pulsation is found at the period predicted by the radio ephemerides with very narrow primary peaks, the width of which is about 1/16 phase ≈ 100μs near the time resolution limit (61μs) of the observation. The pulsed luminosity within the primary peak (1/16 phase interval) is found to befp= 4.0 × 10−12erg s−1cm−2. Although there may be a secondary peak, its statistical significance is too low to identify. Spectra of the whole source region and the primary peak are fitted by power law models to give photon indices of about unity for both.

Detection of pulsed X-ray emission from the fastest millisecond pulsar PSR B1937+21 with ASCA

Detection of pulsed X-ray emission from the fastest millisecond pulsar PSR B1937+21 with ASCA

Detection of pulsed X-ray emission from the fastest millisecond pulsar PSR B1937+21 with ASCA

Detection of pulsed X-ray emission from the fastest millisecond pulsar PSR B1937+21 with ASCA

Individual and integrated pulse properties of PSR B0943+10 involved in the mode-changing phenomenon

The characteristics of the “burst” (B) mode and “quiescent” (Q) mode pulse sequences–long known from studies at or below 103 MHz–are identified at 430 MHz for the first time. An 18-minute, Polarimetrie observation begins with a long Bmode sequence, which has a higher average intensity, regular drifting subpulses, and a preponderance of primary polarisationmode radiation. An abrupt transition to a Q-mode sequence is then marked by a) weaker average intensity, but occasional very bright individual subpulses, b) a complete cessation of drifting subpulses, with disorganized subpulses now being emitted over a much wider longitude interval, and c) near parity between the primary and secondary polarisation modes, resulting in pronounced depolarisation, both of individual pulses and the average profile.